The present invention relates to an optical transmission apparatus which can suppress deterioration of transmission quality due to four wave mixing (FWM) generated in a wavelength division multiplexing optical transmission system using an optical fiber and an optical transmission system.
A wavelength division multiplexing (WDM) transmission system, which transmits a plurality of optical signals with different wave-lengths using a single optical fiber, has been put into actual use as a large capacity optical transmission system. An optical fiber amplifier (hereinafter referred to as optical amplifier) such as an erbium-doped fiber amplifier (EDFA) has a characteristic capable of amplifying a wide wavelength range in one operation. Therefore, the combination of WDM and the optical amplifier allows amplifying a plurality of optical signals with different wavelengths in one operation. Thus, economical and large capacity long distance transmission can be realized with a simple structure.
However, such a WDM light amplifying transmission system increases an optical input level to the fiber, so that a transmission characteristic deteriorates, e.g. due to a nonlinear effect. As examples of such nonlinear effects, deterioration of transmission quality may be due to cross phase modulation (XPM), FWM or stimulated Raman scattering (SRS).
The XPM effect deteriorates a transmission waveform because light signals of two waves or more input to the optical fiber are phase-modulated mutually and are influenced by chromatic dispersion of the optical transmission line. JP-A-07-074699 teaches that chromatic dispersion involved in the generation of the XPM can be suppressed by a chromatic dispersion compensator, which is appropriately disposed on the optical transmission line, and deterioration of waveforms can be avoided.
In a case of SRS, part of a wavelength-division-multiplexed signal light input to the optical fiber acts as excitation light and interacts with signal light of a low frequency in the fiber, and energy moves from the signal light of a high frequency to the signal light of a low frequency. Therefore, deviation of an optical signal-to-noise ratio (OSNR) may be caused depending on a waveband. However, when the intensity of the signal light of each wavelength to be input to the transmission line is appropriately controlled, the OSNR between wavelengths can be made uniform (e.g., Japanese Patent Application No. 2001-167609).
When input signal light having two or more multiplexed wave-lengths is input to the optical fiber, light is newly generated by the nonlinear optical effect, and FWM adds interference noise to the signal light. The FWM occurs in various types of optical fibers, and its intensity (FWM light intensity) depends on a type of optical fiber, an input intensity into the optical fiber, or an interval between wavelengths of a signal acting as pump light. The FWM light intensity increases in proportion to the number of wavelengths multiplexed in the optical fiber. The light intensity of the resulting FWM interference increases as the number of frequencies included in the light by the WDM device increases and the input light intensity increases, resulting in noticeable deterioration of transmission quality. Furthermore, the number of relay points increases as the transmission distance becomes long, which results in more noticeable deterioration due to effects because of an increased amount of FWM.
In the operating state of a conventional WDM apparatus, effects by the FWM interference were not so large because the number of wavelengths being multiplexed and the transmission distance were relatively small. In recent years, as network communication traffic increased, demand arose for an increase in the number of accommodated wavelengths and for increases in the transmission distance. Therefore, the effects by the FWM cannot be dismissed, and when the effects of the FWM become large, it becomes impossible to make transmission.
For a conventional transmission system using the WDM apparatus, a method of making a wavelength of light newly generated by the FWM not to agree with any signal light wavelength is described in Publication 1 (F. Forghieri, “Reduction of Four-Wave Mixing Crosstalk in WDM Systems Using Unequally Spaced Channels”, IEEE Photonics Technology Letters, 6, pp. 754–756, 1994).
A method to decrease generation efficiency of the FWM by inputting excitation light to the transmission line and giving a distributed constant type Raman amplification gain to the signal light to reduce an input intensity of the signal light to the optical fiber is described in Publication 2 (N. Takachio, “32×10 Gbps distributed Raman amplification transmission with 50 GHz channel spacing in the zero dispersion wavelength region over 640 km of 1.55 μm dispersion shifted fiber”, 1999 Optical Fiber International Conference, Postdeadline Paper 9) and JP-A-2001-217781.
A design method to minimize deterioration of transmission quality due to such a nonlinear effect is described in Publication 3 (J. Kani, “Inter-wavelength-band nonlinear interaction and their suppression in multi-wavelength-band WDM transmission systems”, IEEE Journal of Lightwave Technology, vol. 17, November 1999). According to this method, when it is predicted that the transmission quality of signal light is deteriorated by FWM, a nonlinear interaction is decreased by arranging for the intervals between respective wavelengths to become unequal or by mutually propagating the signal lights in opposite directions in a single optical fiber. Thus, among the deteriorations in the transmission characteristics due to the above-described nonlinear effect, one especially resulting from the FWM is minimized. It is also disclosed in JP-A-08-097771 that the generation of the FWM light can be decreased by configuring a system to have an appropriate combination of chromatic dispersion values of the transmission fiber.
However, the technologies described in the above-described publications need an optical coupler and pump light for distributed Raman amplification, resulting in an increase in the system cost. Also, there is a possibility that deviation of the OSNR may be generated because signal light intensity deviation is generated for each wavelength. Furthermore, the unequal interval disposition method of the Publication 3 has a problem in that it is not suitable for large capacity transmission because wavelength multiplex density is limited.